1. Field of the Invention
This invention relates to an acid flux melt process that allows melting of incinerator ash and other solid materials that are normally difficult to dissolve, to be readily dissolved in acid. The melt process is performed at a very low temperature, after which the melt can be dissolved and solutions passed to a solvent extraction process or other process to recover the valuable or contaminating materials from the solution.
2. Background Information
The use of an incinerator to treat combustible wastes from a nuclear facility, such as HEPA filters, results in a dramatic decrease in low level and high level waste volume. The remaining material, the ash, contains most of the metals which would include radioactive materials. These radioactive materials make this waste a low level, and sometimes a high level, radioactive waste that must be disposed of, usually at great cost. One method for reducing this cost at facilities where the radioactive material has value, that is a nuclear fuel manufacturing facility, is to remove the radioactive material for recycle to the process. If this removal is complete, or nearly complete, then the remaining ash can be disposed of as non-radioactive waste at a much reduced cost. In addition, the value of the recovered materials in the waste, which may be of substantial value, is not lost.
Many methods have been tried for recovering materials from incinerator ash. However, this ash is usually in a crystalline/glassy oxide form which is very difficult to dissolve. Some have attempted leaching with strong acids. These attempts have not resulted in complete recovery. For example, repeated leaching of uranium containing incinerator ash has reduced initial uranium levels from the 10% to 40% level to the 6,000 ppm (parts per million) U range. Pre-grinding has not improved this residual uranium level. Since residual levels of less than about 7 ppm U are required for the leached ash to be classified as clean, this ash must be buried as low level radioactive waste at great expense. In addition, the residual uranium values are lost and the large quantities of contaminated leach solution must still be processed at some cost. One such method, taught by F. G. Seeley, et al. in Oak Ridge National Laboratory Report ORNL/TM-8913 "Development of Processes For The Solubilization of Uranium From Waste Leach Residue." Abstract and pp. 21-25, March 1984, utilizes a cal-sinter process, where CaO is used as a sintering media. In the process, the CaO reacts with refractory metal silicates at 1200.degree. C., and provides subsequent solubilization of uranium from the sinter matrix by an acid leach. Another process employs fluoride in the sinter media to free the uranium from refractory silicate at a lower temperature of 700.degree. C. to 900.degree. C., so that the uranium is soluble in subsequent acid leaching.
The acid resulting from these leaching processes is then treated to recover the valuable components. Leach acid processing to recover the valuable components of a feed solution can involve extraction, scrubbing, stripping and precipitation steps, as taught in The Nuclear Fuel Cycle, ed. P. D. Wilson, Oxford University Press, pp. 33-46, (1996), and Separation Science and Technology, "Modeling Of The Simultaneous Extraction of Nitric Acid And Uranyl Nitrate With Tri-n-butyl Phosphates.cndot. Application To Extraction Operation." Jozef J. Connor et al. 34(1), pp. 115-122, (1999). Other well known processes to recover uranium and other metal values from a variety of starting materials include U.S. Pat. No. 5,045,240 (Skriba et al.) were leaching in a fluidized bed and U.S. Pat. Nos. 4,446,114 and 4,430,309 (Jardine et al. and York, respectively) relating to sulfuric acid or nitric acid addition to a scrub, strip or wash step during the subsequent solvent extraction step.
Other methods have also been tried. The most successful has been dissolution in molten caustic (NaOH). While resulting in complete dissolution of the incinerator ash into a melt, this melt has a relatively high melting point, 594.degree. C. (1100.degree. F.) or greater, depending on the amount of silicon and aluminum present and the amount of carbon dioxide absorbed. Another difficulty results from the aluminum metal that may be present. Aluminum is a common component of incinerator ash from nuclear facilities and results from the HEPA filters that are commonly incinerated to reduce their volume. The aluminum metal remains in the ash and reacts with the caustic to produce hydrogen gas, which can explode. This characteristic certainly is not desirable in a nuclear facility and is considered a severe safety hazard. One such caustic dissolution method is taught by H. L. Chang et al. in Ind. Eng. Chem. Res., "A General Method For The Conversion Of Fly Ash Into Zeolites As Ion Exchangers For Cesium" 37, pp. 71-78, (1998), where fly ash from utility power plants was fused with NaOH, at the temperature of 550.degree. C., followed by dissolution in water and a hydrothermal treatment.
What is needed is a low temperature process that results in superior uranium recovery in a cost effective manner and does not produce hydrogen or other undesirable off-gases.